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Clock & Timing

March 17, 2011

As you know, if you want a car that you can drive at 150 MPH, then you will pay a premium since it will require additional technology to keep you connected to the road and overcome the frictional forces of the air - as well as the "Gee, I look really cool in this car" effect which at times comes with an even greater price tag. In the physical world of high speed data and signal integrity, these laws also apply. Dr. Howard Johnson knows this well and has published several books on the subject. Even the subtitle "Advanced Black Magic" implies the difficulty in designing high speed systems.

Well folks, it isn’t getting easier. In fact, it is getting far worse. What is interesting about our world is our fundamental quest for knowledge - and the more rich the content of the information, the quicker people learn or share information. There is also the desire to communicate and the later also applies... the richer the content (photos, videos, music, etc) the more appealing the media. With the passing of the DMCA Title 2 which protected service providers from copyright infringement when making local copies to stream (or the unscrupulous pirates stealing it from them) along with the deployment of DOCSIS modems (now version 3.0 exceeding 100Mbps up and down - if the OSP is willing) the stage is set for one of the largest bandwidth explosions ever witnessed by man.

This expansion of bandwidth is driving data center equipment to ever increasing capacities... it wasn’t long ago that 1Gbps was fast... not any more. The norm in data centers now is 10Gbps Ethernet (802.3ae - optical) and quickly moving to 100G Ethernet! The latter has been accomplished via 10 lanes of 10Gbps, but is moving to 4 lanes of 25Gbps which matches the number of lasers and receivers found in most 100G modules. Do you know what happens to a 25G signal when it travels over a back-plane... it isn’t pretty. In fact, 10G has issues as well and it’s amazing that it works at all...

For example, take a look at the image below. This is a comparison of PCI Express signals (generation 1 through 3) over 26 inches of differential traces on a PCB (FR-4). As the speed of the signal increases the eye opening decreases. What used to work without issue now requires either a change in board material or active circuitry to restore the signal. These signals are far slower than a 25-28Gbps stream now being considered for electrical interface to optical modules. Without signal conditioning, careful layout (thank you Dr. Johnson), and good impedance control... no bits, just noise...

If you want to know more about fixing this, visit http://www.national.com/datacom and watch some of the cool videos on how it’s done... as Spock would say... "Fascinating"... Till next time...